The invention relates to the general field of telecommunications.
It more particularly concerns the signaling, in a cellular telecommunications network, of resources allocated to a terminal to communicate over this network.
The invention thus has a favored, but not limiting, application in the context of LTE (Long Term Evolution) cellular mobile telecommunications networks as defined by the 3GPP (Third Generation Partnership Project), and notably on the downlink, i.e. in the direction of communication from the base station (or eNodeB) toward the mobile terminals (or UE for User Equipment).
In a known manner, the capacity of cellular mobile telecommunications networks, and in particular that of LTE networks, is limited by interference. This interference can be of different natures. Among the most damaging in terms of cellular network capacity, are                SU-MIMO interference (for Single User-Multiple Input Multiple Output) related to the use of multiple transmitting and receiving antennas, and which corresponds to the interference generated between MIMO data streams allocated to one and the same terminal;        MU-MIMO interference (for Multiple User-Multiple Input Multiple Output) which corresponds to the interference generated between MIMO data streams allocated to different terminals; and        Intercellular interference, generated between signals emitted by different cells and intended for different terminals.        
Various methods making it possible to reduce the effect of this interference on the performance of the network are known to the prior art.
Thus, interference of SU-MIMO and MU-MIMO type can notably be treated using a precoding technique applied to emission. This method consists in the application of complex weightings to the streams of data emitted by the antennas of the base station so as to attribute particular spatial properties thereto (for example a favored direction). Thus, for example, via this precoding it would be possible to separate in space the streams intended for one and the same terminal (in the SU-MIMO case) or to different terminals (in the MU-MIMO case) in order to allow the receiver terminal to detect each stream with a reduced interference from the other streams.
The intercellular interference can be reduced via the use of so-called CoMP (for Coordinated MultiPoint) techniques, aiming to coordinate the allocation of resources and the precoding implemented by the base stations controlling the neighboring cells of a given cell, so as to minimize the interference created on this cell by its neighbor cells. These techniques are described in more detail in the document TS 36.819 titled “Coordinated MultiPoint operation for LTE physical layer aspects”, Release 11, edited by the 3GPP.
The most effective precoding techniques used to reduce MU-MIMO interference and/or intercellular interference require precise knowledge at the emitter of the transmission channel of the different terminals. Such knowledge is generally not available at the base station (eNodeB), because it requires considerable signaling on the return link between each terminal and the base station.
In order to limit this signaling, the information relating to the knowledge of the channel is generally quantified. There follows a loss of precision so that the interference is not in practice totally suppressed by precoding: a non-negligible proportion of residual interference subsists, which affects the terminal performance.
To remedy this drawback, it is known to use, at the terminals, non-linear receivers implementing interference cancellation, such as for example MMSE-SIC (for Successive Interference Cancellation) receivers. In general, for the sake of simplicity in the case of a single interferer, such a receiver estimates the interfering stream of data (corresponding to interference of MU-MIMO or intercellular type) for example by implementing a step of (channel) decoding of the signal of the corresponding interferer. Then based on this estimate of the stream, on the estimate of the channel of the interferer and on the knowledge of the transmission parameters allocated to this interferer, the receiver reconstructs the interfering signals received by the terminal. The reconstructed interfering signal is then subtracted from the signal received by the terminal, the signal thus cleansed of the interference being then used to detect the useful signal intended for the terminal.
SIC non-linear receivers can process one or more interfering data streams intended for one or more so-called interferer terminals. However, as stated previously, the process of cancelling the interference implemented by these non-linear receivers requires the knowledge of the transmission parameters of the interferer(s) and particularly, in the context of an LTE telecommunications network, of the modulation and coding scheme associated with each interferer, of the physical resource blocks (PRB), where applicable of the dedicated pilot sequence (known as DMRS for DeModulation Reference Signal) if it is used, and of the RNTI (Radio Network Temporary Identifier) allocated uniquely to the interferer terminal to identify it on the cell to which it is attached.
For these reasons, MMSE-SIC receivers with interference channel decoding are commonly envisioned to process SU-MIMO interference, since the terminal equipped with the MMSE-SIC receiver has access to all the transmission parameters of the various streams that are transmitted thereto. On the other hand, their use for processing MU-MIMO and downlink intercellular interference is more complex.
To better illustrate this statement, it is advisable to recall how the allocation of transmission parameters in an LTE network is carried out, and more particularly how the signaling of the transmission parameters thus allocated is carried out.
For the sake of simplicity, only intercellular interference will be addressed, knowing that equivalent statements also apply to MU-MIMO interference.
The transmission parameters are allocated to each terminal by the base station controlling the cell to which the terminal is attached. They are, with the exception of the RNTI identifier, communicated to each terminal via a dedicated PDCCH (Physical Downlink Control Channel). The RNTI identifier is signaled to the terminal in a dedicated signaling message, and more specifically in a configuration message transmitted over the PDSCH channel (Physical Downlink Shared Channel) managed by the upper layers of the network, and particularly by the RRC (for Radio Resource Control) layer.
The PDCCH channel is organized according to several possible formats, known as DCI (for Downlink Control Information) formats. A DCI format includes several fields, each field bearing a particular item of information (e.g. transport block or PRB (Physical Resource Block) allocated (one or two transport blocks can be allocated) MCSs (for Modulation and Coding Scheme) allocated for each transport block etc.). The information bits of a PDCCH channel (i.e. the bits of the DCI format) are then associated with a CRC (Cyclic Redundancy Check) code to allow error detection. The peculiarity of this CRC code is that it is scrambled with the RNTI identifier of the terminal to which the PDCCH channel is intended. This allows the terminal to validate that the PDCCH channel it is decoding is indeed addressed to it Specifically, if another terminal (which possesses a different RNTI) attempts to verify the validity of the PDCCH channel using this other identifier RNTI, the verification of the CRC code returns an error thereto.
The information bits as well as the bits of the CRC are then encoded using a convolutional code, then scrambled by a sequence specific to the cell, before being modulated in QPSK then transmitted. The effective rate of the convolutional code, which depends on the rate of the code as well as the data rate adaptation carried out at the output of the encoder to adapt the number of coded bits to the resources available, is adapted to the protection level required by the radio conditions of the terminal to which the PDCCH channel is intended. Thus, a PDCCH channel intended for a terminal that is in good radio conditions (for example close to the base station serving it) does not need much protection and is transmitted with a high effective rate, or in other words over a low number of resources. Conversely, a terminal in poor radio conditions, is allocated a PDCCH channel with a low effective rate, and occupying a higher number of resources.
In LTE networks, the resources occupied by a PDCCH channel allocated to a terminal are not known in advance by the latter. The terminal must therefore test a set of possible resource combinations, and for each candidate combination attempt to decode a PDCCH channel potentially transmitted over these resources with its RNTI identifier, to determine if it is intended for it. All the candidate resources for a given terminal is called a search space, and described in document 3GPP TS 36.213 v11.0.0 titled “Evolved Universal Radio Access; Physical Layer Procedures (Release 11)”, September 2012, in section 9.1.1 in particular. The position of the search space depends in particular on the value of the RNTI.
It will thus be understood that the only possible solution for allowing a terminal suffering intercellular or MU-MIMO interference to acquire the knowledge of the transmission parameters of its interferers in order to be able to implement an MMSE-SIC processing method to cancel this interference, is to attempt to decode in a blind manner all the PDCCH channels of the interfering cell(s) (i.e. of the cells serving interferer terminals for the terminal equipped with the MMSE-SIC receiver).
In other words, for each interfering cell, the “victim” terminal must examine each PDCCH channel likely to have been transmitted over resources of search spaces corresponding to different values of RNTI, until all the PDCCH channels effectively transmitted are found. As the victim terminal does not have knowledge of the RNTI identifiers allocated to the other terminals, a test of all the possible RNTI values must be carried out. Once all the PDCCH transmitted by the interfering cell in question are decoded, the victim terminal can know which are the terminals of this cell that are served on the same resources as it and which represent interferers for it. In addition, the victim terminal has access to the transmission parameters of its interferers and can thus cancel their interference using an MMSE-SIC technique.
This solution has two major drawbacks.
First of all, the blind decoding of the various PDCCH channels of a cell is a very complex and particularly long operation due to the processing implemented. It is moreover a great consumer of energy of the terminal battery. The further application of such a blind decoding to several interfering cells to allow the use of a MMSE-SIC processing with a view to eliminating the inter-cellular interference is all the harder to envision.
In addition, the need to test for each candidate PDCCH all the possible values for the RNTI identifier (there are 216), in order to determine both the latter and the validity of the candidate PDCCH, renders this method almost impossible to implement with realistic computation means.
The document WO 2010/108136 proposes a method making it possible to facilitate the cancellation of intercellular interference in a mobile telecommunications network, and particularly in an LTE network.
This method consists in dividing all the RNTI temporary identifiers available for a cell into two subsets: a first subset of RNTI identifiers (that will be named in “open” RNTIs in the remainder of this document) intended for terminals liable to cause interference, and a second subset of RNTI identifiers intended for terminals liable to suffer interference. The open RNTI identifiers of the first subset are published (i.e. broadcast), so as to be known to the terminals of the neighbor cells. Each open RNTI identifier uniquely identifies a terminal.
Thus, a base station of a cell wishing to perform the transmission from a PDCCH channel toward an interferer terminal decodable by a terminal of a neighbor cell can use to do this an open RNTI identifier. The decoding of the PDCCH channels by a terminal that is a victim of interference to acquire the transmission parameters of its interferers is therefore facilitated since it can be limited to open RNTIs published by the cells, as well as to the corresponding search spaces.
Document WO 2010/108136 proposes to signal to a terminal that one (or more) open RNTI identifier has or have been allocated to it, by sending this open RNTI identifier to it via the PDCCH control channel or via a shared physical channel or PDSCH (Physical Downlink Shared CHannel). On receiving this identifier, the terminal in question can than decode the PDCCH channels that are intended for it using the open RNTI identifier that has been allocated to it.
Such a signaling method can turn out to be costly in terms of signaling resources, particularly when a large number of open RNTI identifiers is allocated, each RNTI being coded on 16 bits as per the LTE standard. Specifically, a signaling of an open RNTI via a PDCCH channel involves adding the 16 bits of the open RNTI to the DCI format of this PDCCH channel, which diminishes the coverage of the PDCCH channel, particularly when the terminal is at the cell edge and is already using the most robust PDCCH format (i.e. that which occupies the maximum number of resources that can be allocated to a PDCCH channel).
This is all the more so when the “interferer” status of a terminal in the same may as the resources that are allocated to it are likely to evolve rapidly over time, and thus to require a high degree of signaling.
Moreover, as mentioned previously, according to this method, an open RNTI identifier uniquely identifies an interferer terminal. However, the number of terminals served by a base station of a cell can be high (several tens or even hundreds of terminal). The allocation of a large number of open RNTI identifiers by a base station of a cell thus leads to high computational complexity for the decoding of the associated PDCCH channels, as well as high energy consumption for the terminals implementing a successive interference cancellation method.